This invention relates in general to heat transfer apparatus for evaporating, distilling, freezing or chilling liquids, and more specifically, to an orbital drive for a whip rod used in conjunction with a vertical tube type of heat transfer apparatus.
In the processing of liquids it is often required to evaporate the liquid, as in the production of fresh water from seawater, in distillation processes, and in the production of ice slurries and other cooled or slushy liquids. Ice slurries are useful, among other applications, for cold storage to reduce peak load power demands in building air conditioning systems and to provide refrigeration for food such as fish catches stored on fishing vessels.
Traditional evaporation and distillation apparatus, without or without vapor compression, require high energy inputs to heat fluids and drive compressors. Also, since evaporation or condensation occurs primarily at the interface between the liquid and its vapor, heat must traverse a container wall and a layer of the liquid to reach this interface. As a result, there is a substantial temperature gradient per stage in conventional evaporation apparatus. This limits the number of stages that can be provided for a given temperature rise, or requires a greater energy input.
U.S. Pat. Nos. 4,230,529 and 4,441,963 issued to the present applicant disclose a new approach to solving these problems. It involves using a vertical, thin-walled, open-ended heat transfer tube (or tubes) which is driven in an orbital or wobbling motion. This orbital tube motion increases the heat transfer efficiency by reducing the thermal resistance at the inner and outer surfaces of the tube. The motion swirls the liquid to be evaporated into a generally thin film over the inner surface of the tube. This increases the evaporation surface area and decreases the thermal resistance by decreasing the thickness of the liquid layer. The orbital motion also aids in heat transfer into the tube at its outer surface produced by condensation of a heated vapor stream. The condensation increases the thickness of the liquid layer at the outer surface, and hence its thermal resistance. The orbital motion throws off the droplets, thereby increasing the heat transfer at the outer wall.
Both of these patents teach multiple such tubes held in a common container and driven by eccentrics to underdo a wobbling motion in a horizontal plane. The liquid is driven in turn by a dynamic coupling to revolve over the inner surface as it flows down the tube under the influence of gravity. These arrangements require cranks, bearings and complicated seals inside the evaporator. The component parts are difficult and costly to manufacture and assemble, they must be machined to close tolerances, they are susceptible to corrosion and contamination when used in the chemical industry, and they wear which leads to a deterioration in the balance of the wobbling tubes and attendant vibrations. The '529 patent also discloses a self balancing arrangement with a self adjusting orbital radius that accommodates the balance to changes in mass. If the base moves, the crank radius, however, must be fixed, and even this step may not be adequate.
Many known heat transfer devices ranging from ice cream makers to sophisticated evaporators use a rigid wiper bar that is positively driven to rotate within the tube to spread viscous liquids into a thin, evenly distributed film. Positively driven wipers can handle fluids with a viscosity of 1,000,000 c.p. or higher. (Water has a viscosity of 1 c.p.) However, any rigid, positively driven wiper or scraper has drawbacks. First there is a need to introduce and seal a rotational drive shaft. Second, because the wiper or scraper is rigid and moving over a fixed surface at close spacings, manufacturing and assembly become difficult and costly. The surface must be machined to close tolerances, as well as the wiper/scraper and its support structures. Further, these rigid wiper arrangements are susceptible to, and comparatively intolerant of, wear.
To solve these problems for less viscous fluids, e.g. those with a viscosity of 1 to 1,000 c.p., the '399 patent describes a whip rod located in the tube which spreads the feed liquid into a highly thin and uniform film to reduce its thermal resistance and to enhance its evaporation. The whip rod also controls the build up of solid residue of evaporation. The '399 patent discloses several arrangements for mounting the rod, including lengths of cables, a flexible, but non-rotating anchor connected between a base and the lower end of the rod, and a double universal joint also connected between the lower end of the whip rod and the base. While the whip rod is effective as a film distributor, the mounting arrangements have disadvantages. They increase the overall material, assembly and operating costs. Also, they fail. Material fatigue of flexible cables supporting the whip rods is a particular concern.
U.S. Pat. No. 4,762,592 describes an orbital drive that overcomes the manufacture, assembly, wear and balance problems of the earlier eccentric-crank drives. This improved drive uses a rotating counterweight or weights mounted on the evaporator and a spring-loaded strut suspension for the evaporator. The counterweights and the mass of the evaporator revolve around one another as the counterweights rotate.
While this arrangement does overcome the problems associated with an eccentric crank drive, it also suffers from certain deficiencies. For example, it requires the orbital movement of a large mass, particularly where the unit is scaled up to a commercial size with multiple large tubes, each carrying a liquid stream. This mass increases the power requirements (particularly on start up), increases the demands on the spring-strut suspension, can lead to an early fatigue failure of the suspension, and generally increases the construction and operation cost of the system. It also increases the desirability of a stable operating platform, e.g. a concrete floor, as opposed to one that moves such as a ship at sea or some other transport. While the '592 patent proposes a solution to the moving platform problem, the solution in practice has not been adequate when the apparatus has been scaled up to commercially useful sizes.
Another design consideration is the requirement in certain applications to maintain the fluid being processed in a totally sealed environment, e.g. one that is aseptic. But because a heat transfer device uses continuous processing, there are seal and other problems in conducting the fluids flowing between the orbital system and the fixed surrounding environment and in introducing motive force. Conduits must be flexible to at least a certain degree to accommodate the orbital motion, but maintaining this flexibility becomes increasingly problematic as the system is scaled up in size. Ideally there are no sliding rotary seals since they are difficult to maintain with the required degree of reliability in the seal, particularly as they wear.
While the orbital tube approach has been used for evaporation and distillation, heretofore it has not been applied for freezing. One reason is that the liquid freezes to the heat transfer surface and greatly reduces any performance advantages of the orbital tube approach.
It is therefore a principal object of this invention to provide an orbital drive for use with vertical heat transfer tubes that drive a comparatively small mass, has lower power consumption as compared to known orbital tube systems, and which can operate readily on a moving platform.
Another principal object is to provide these results with an apparatus that can be readily scaled up in size.
A further object is to provide the foregoing advantages without critical machining of parts or carefully controlled assembly.
A still further object is to provide the foregoing objects with while being substantially insensitive to wear of parts in the drive.
Another object is to provide the foregoing advantages while also providing compatibility with simple and reliable arrangements for distributing the liquid being processed to multiple tubes.
A further object is to provide the foregoing advantages while also sealing the fluid being processed, including sealing without sliding rotary seals.
Another object is to provide the foregoing advantages for heat transfer equipment used for evaporation, distillation, chilling and freezing.